The invention relates to determining the minority carrier lifetime in the depletion layer of a semiconductor wafer.
The performance and reliability of semiconductor electronic and optoelectronic devices, and the integrated circuits into which they are incorporated, depends in part upon the purity of the semiconductor from which the devices are made, and, in particular, on the level of contaminants or impurities which may be introduced during manufacture and processing. One measure of the level of impurities used in quality control is the determination of the minority carrier lifetime. When a minority carrier is introduced into a region where carriers of opposite polarity (i.e, majority carriers) are present, there is a tendency for recombination and a consequent annihilation of the minority carrier. The lifetime of a carrier depends on how many impurities or defects which form sites where the recombination occurs are present. The distance that the carrier travels during their lifetime is called the diffusion length. Both the lifetime and the diffusion length provide a measure of the impurity or defect concentration.
There are two types of minority carrier lifetime, the recombination lifetime .tau..sub.R (discussed above) and the generation lifetime .tau..sub.G. These lifetimes are distinguished based on their relationship to electron-hole recombination and electron-hole generation, respectively.
In silicon, both of these processes are governed by impurity or defect levels E.sub.T. In a p-type semiconductor, the recombination lifetime is related to an annihilation of electrons (excess minority carriers). The recombination is very efficient in the bulk region where free holes (majority carriers) are present. The generation lifetime relates to an opposite process. That is the creation of the minority carriers due to thermal generation. Again, the impurities and defects serve as sites for the generation. The generation lifetime is important in the depletion layer where virtually no majority carriers are present and thus the recombination is very inefficient.
The depletion layer generation lifetime is ideally suited for monitoring impurities or defects in these semiconductor layers such as epitaxial layers and denuded zones which are extensively utilized in semiconductor microelectronic devices and circuits.
Determining lifetime using surface photovoltage (i.e., the change of the surface potential caused by illumination) techniques has been limited to measuring the recombination lifetime, including the bulk recombination lifetime .tau..sub.R, the effective recombination lifetime .tau..sub.eff which contains contributions from recombination in the bulk and at the surface; and the surface recombination lifetime .tau..sub.s. SPV surface recombination lifetime measuring tools are being produced by Semitest, Inc., Billerica, Mass. and QC Solutions, Woburn, Mass. Both of these tools perform single frequency, room temperature SPV measurements.
Measurement of the depletion layer generation lifetime in thin epitaxial layers, on the other hand, has been limited to those techniques which utilize test junctions or test MOS capacitors. Another method uses a corona charge pulse applied to a small site on the surface and determines the generation lifetime from the contact potential transient following the pulse. These approaches use capacitance transient measurements in response to a bias pulse applied to the junction or to the gate of the MOS device (see Kang et al., The Pulsed MIS Capacitor, Phys. Stnt. Sol. 89:13, 1985).